This invention is concerned with electroacoustic transducers and, more specifically, with electroacoustic transducers for transmitting or receiving sound in a fluid medium. Although not limited to the ultrasonic frequency region, this improving the performance characteristics of electroacoustic transducers to be used in the ultrasonic frequency region in a gaseous medium.
The teachings of this invention can be used by one skilled in the art in a wide variety of transducer designs using many different methods of transduction for sound radiation or reception in fluid or gaseous mediums. Transducers using the teachings of this invention can be designed using a wide variety of transduction materials, such as magnetostrictive rods, piezoelectric crystals, and polarized ceramic elements. However, the teachings of this invention can be best used in conjunction with ultrasonic transducers designed with polarized ceramics for use in a gaseous medium. Such a transducer is described in U.S. Pat. No. 3,928,777.
U.S. Pat. No. 3,928,777 described an ultrasonic transducer employing a ceramic disc as the transduction material. For optimum performance, the transducer is operated in the vicinity of resonance. This disc could operate in the thickness mode of resonance, but in the preferred embodiment the ceramic disc operates in the radial mode of resonance, since a transducer of this design is smaller and less costly. The transducer further employs a novel acoustic transformer in the form of an acoustic impedance matching material inserted between the ceramic and the gaseous medium. This acoustic transformer material is characterized in that its acoustic impedance is less than the acoustic impedance of the ceramic, but greater than the acoustic impedance of the gaseous medium. The acoustic impedance of a material is its density, ρ, times the velocity of sound, c, in the material. In addition, the thickness of the acoustic transformer material is approximately one quarter of a wavelength.
As shown in the referenced patent, a transducer utilizing the teachings of that invention will produce significant increase in sensitivity over a broader frequency response, and the resultant acoustic radiation pattern will invention is particularly useful for contain reduced secondary lobes. However, the beam angle from the transducer is fixed and controlled at the resonant frequency by the diameter of the ceramic disc. To overcome that limitation, this invention provides a novel design of an acoustic transformer with dimensions that are not the same as the dimensions of the ceramic disc. This acoustic transformer can therefore be designed to produce a wide variety of different acoustic radiation patterns. These radiation patterns can be conical in shape with different beam angles, or they can be fan shaped containing one beam angle in the horizontal plane and a different beam angle in the vertical plane. The transducer can also be designed to utilize a plastic housing that provides improved environmental protection.